Jessica congratulated her younger sister and f(x) member Krystal on her birthday.

Showing some real sisterly love, the older Jung sister took it Weibo on Sunday, October 25 to greet Krystal.

In the pictures, Jessica and Krystal are making funny faces at the camera, showing the close bond the two have with each other.

"Happy birthday my baby," Jessica said. "Don't grow too fast..."

Krystal celebrated her birthday on October 24 and was soon followed by the release of f(x)'s fourth album 4 Walls on Tuesday.